## What We'll Be Building
## HTML and CSS
Every website, application, form or component starts with markup. The HTML will change over time as you develop, but a first pass helps you understand the UI you are trying to build.
## Demo
In this exercise we will scaffold out some HTML for our todo app, then add some basic styling to it.
### Page scaffold
```html
```
1. The [`DOCTYPE`](https://developer.mozilla.org/en-US/docs/Glossary/Doctype) tells the browser that this file is written in modern HTML.
2. The `html` tag wraps the entire page and is the page root. Nothing is placed outside of this tag.
3. `head` will contain all of the page's metadata, in this case a link to our CSS file.
4. `body` is where all of the visible content should be placed.
### Content Sectioning
As we saw in the previous demo, HTML elements can be used to describe different content sections of the applications. Let's add `header`, `main` and `footer`, as well as populate the header with an `h1`, addTodo `div`, and `nav` for our filters.
```html
```
> Note that a `form` element would have been more semantically correct than a `div`, but we aren't using this form to POST to a server, so for this example a div is easier to use.
### Updating the header
The header of our page is where most of the action will happen. First, let's give our app a name, adding "TODO" to our `h1`. Then we can add an input and button to our `addTodo` div.
```html
```
#### Navigation
The navigation for this application is quite simple. We want users to be able to switch between three filtered states. Since we need to track which state is currently selected, we'll add a `selected` class to the first item.
```html
```
### Adding styles
Now that we've got the top of our application scaffolded, we can add some styles in the `head`.
```html
```
### Updating styles
It looks like the selected button isn't getting any special styles. Let's dig in and see why that is.
Open up the browser inspector and target our "all" button. You'll notice that the blue style is present on the list, but it is being overridden by the `border: none` above it. This is a situation where specificity is winning out over the cascade.
> **Cascade** states that if two selectors are equal, the lowest one on the page wins
> **Specificity** states that regardless of cascade, the selector with the highest specificity wins
To fix this problem we need to either reduce the specificity of our button styles, or increase the specificity of the selected style. In this situation we will add `.filter` in front of the `.selected` selector, because the selected style only applies to the filter anyway.